Method of pumping liquefied gases containing nitrogen



Dec. 8, 1953 F. E.-PAv|.ls 2,661,608

METHOD 0F PUMPING LIQUEEIED GASES CONTAINING NITROGEN Filed May l5. 1948 MAMMMMAM /NI/ENTOR ATTORNEY Patented Dec. 8, 1953 r orric METHGD OF PUMPENG LIQUEFHED GASES CONTAINNG NTRGEN Frank E. Pavlis, Allentown, Pa., assigner to Air Products, Incorporated, Emmaus, Pa., a corporation of Michigan Application May 15, 1948, Serial No. 27,2613

6 Claims.

In the operation of plants producing oxygen and nitrogen by the liquefaction and fractionation of air, it is often desirable to provide a supply of nitrogen at a pressure considerably higher than that at which it exists as a separated product at any point in the fractionating cycle. Heretofore such supplies have been obtained by recompressing the final gaseous product nitrogen, which normally comes from the warm air interchanger at substantially atmospheric temperature and. pressure.

'Ihe recompression of expanded gaseous products to high pressures is in severa-1 respects a wasteful and undesirable process. It requires multistage compressors which, if oil-lubricated, contaminate the recompressed gas with hydrocarbons, while if water is used for lubrication it is usually necessary to redry the compressed gas by passage through an adsorbent such as silica gel. Therefore the total plant required is costly, occupies considerable valuable space, and has an unduly high power consumption.

I have discovered that a material and useful portion of the nitrogen content of the air supplied to an air fractionating operation may be withdrawn, in liquid form or as a vapor subject to liquefaction; that this withdrawn portion may be cooled, or condensed and cooled, to a temperature at which it may be pumped in liquid form without risk of gas-locking the pump, and that .i

the refrigeration required for such cooling or condensation and cooling may be produced through the use of higher-boiling column products by maintaining suitable pressure differences between the cooling agent and the liquid to be cooled or the vapor to be condensed and cooled.

By pumping in liquid form to the higher pressure required for storage in pressure tanks or cylinders or for delivery into pipe lines, and vaporizing and warming the high-pressure liquid by heat interchange with the entering air, material advantages are gained at the cost of only the amount of refrigeration represented by the difference between the heat contents of the gas in compressed and in expanded form. These advantages include great economy in plant and 'floor space in the substitution of a simple (usually plunger-type) liquid pump for the elaborate recompressing station, and the complete avoidance of contamination, such pumps needing no liquid lubrication. Safety is increased because of the reduction in the size and the temperature Ylevel of the compressing means and the freedom from contamination by lubricants. are usually desirable reductions in power con- In addition, there (Cl. (i2-175.5)

2 sumption, operating attention, and maintenance.

The invention may be practiced in the operation of either a single-stage or a two-stage co1- umn in the manners illustrated in the attached drawings and the following description thereof, in which:

Fig. 1 is a flow sheet of a single-column operation in which the nitrogen is withdrawn as a Vapor from a preliminary partial fractionation and is condensed and cooled by the gaseous product nitrogen from the column, and

Fig. 2 illustrates in a similar manner an operation in which liquid nitrogen is withdrawn from the high-pressure stage of a two-stage column and is cooled by expanded high pressure nitrogen product from the high-pressure stage.

Referring first to Fig. 1, the essential elements oi apparatus are: the primary interchanger A, which must have four flow circuits but otherwise may be any one of the many conventional types; a nitrogen separator or preliminary fractionator B, which may be either a packed or a bubble-tray column; a condenser C of the shell-and-tube or shell-and-coil type providing reflux for this fractionator; a single-stage fractionating column D, which may be of the packed type but preferably is provided with bubble trays; a nitrogen condenser E, arranged around or within the lower end of the main column D, a nitrogen subcooler F, which is basically a two-iiow circuit interchanger but is frequently built as a three-flow interchanger for reasons of better operation or flexibility; and a pump G, adapted to handle highly Volatile liquids.

The cycle illustrated in Fig. l functions in the following manner: Air under a desired relatively high pressure, for example from 500 to 2500 pounds per square inch gauge, and at least substantially freed from vapors of water and carbon dioxide, is introduced at Il) into a coil I in interchanger A, in which it is in counterflow heat transfer with product nitrogen and oxygen and with the pumped nitrogen stream. Leaving the interchanger through conduit I2, the stream of partially refrigerated air is expanded in passing through expansion valve I3 to an intermediate pressure which for example usually Varies from pounds to 20) pounds per square inch gauge. This expansion liquees part of the air, a portion of which enters separator B as a mixture of liquid and saturated vapor.

The oxygen-enriched air collecting in liquid pool id in the base of the separator passes through conduit l5 and an expansion valve i6, by which its pressure is still further reduced, for example to l pounds per square inch gauge or less, into a coil I1 in condenser C. The jacket of this condenser is supplied with nitrogen vapor through conduit I8, the condensate returning at |9- to supply reflux liquid to the column. The oxygenenriched air stream then flows through conduits 2G and 2l and stop valve 22 into column D at a medial height, as at 23.

In this and the following description, it will be understood that valves mentioned by number are open unless otherwise stated.

The portion of the nitrogen vapor that is not liquefied in condenser C passes through conduit 2li to nitrogen condenser E, in which it is liquefied by heat interchange with pure oxygen boiling in a pool 25 in the base of column D. The resultant liquid 'lows through conduit 2f, stop Valve 2l, and conduit 23 to a coil 29 in interchanger F, in which it is subcooled in a manner later described. The subcooled liquid stream passes through conduit and valve 32| to the suction side of pump at which point Ait is still at substantially the intermediate pressure. Being raised by the liquid pump vto a ldesired higher pressure, the stream `flows through lconduit '32 to coil 33 of interchanger A, in which the liquid is vaporizcd and -the vapor brought to atmospheric-temperature by hea-t interchange With the entering air feed. `The gaseous stream is finally discharged at Si, at the elevated pressure produced by the liquid pump, into cylinders, a storage tank, or a pipe line, not shown.

lThe low-pressure gaseousV nitrogen separated lin column D flows through conduits i5 and at Vand Valve -l' to a jacket KS surrounding the cold end of pump G, in which vit functions to absorb heat entering the cold portion of the pump vthrough Athe insulation or vgenerated within the pump as friction, and thence through conduit it to the shell E0 of inter-changer A, in which it is brought to atmospheric temperature and is vented from the cycle at l The pure oxygen separated in the column iiows through conduits 52, 53, and 55 and valve 55 to `a coil 55 in interchanger A, from which Vit is vented at 5l at substantially `atmospheric ternperatureand pressure. When the apparatus represented by Fig. l is used to .produce oxygen, andno compresse,dnitro-` een is desired, the ,nitrogen ,stream 'liquefied in condenser lf3 passes .through nitrogen suhcooler F, through @enduits .3@ and i2 .and V@massimi valve 43 to the top of the column D as at 5154, where it functions as a useful reilux liquid.

As mentioned above, the liquefied stream .of nitrogen is subcooled in the interchanger F asV it otvs through the coil 29 on Yits Way to the suction side `of the pump. This subcooling is accomplished by passing a stream Vof .gaseous product nitrogen to the shell of the interchanger F in heat exchange relation With the coil 28. For this purpose a portion of the gaseous nitrogen produ ct stream vented from the column D by the conduit 45 is .conducted past .a valve 53 and through conduits 59, E and 6i into the shell ,52 of the interchanger F. After passing heat exchange relation YWith the coil 2,9, the stream of gaseous product nitrogen leaves the shell 76,2 by way of a conduit lil from whence it is conducted through conduit S3 to :the conduit 49 :Where it merges with the stream of gaseous product nitrogen leaving the pump jacket 'd8 on its Way kto .the main interchanger A. The valves 4l and 58 are adjusted to provide the necessary flow of 4gaseous product nitrogen through the interchanger F to provide the desired degree of subcooling.

Referring now to Fig. 2, the essential elements of apparatus are: the primary interchanger H; an expansion engine l, which may be of the reciprocating or the turbine type; a two-stage fractionating columnJ, which may :be of any iconventional or preferred form; a two-pass heat interchanger K used as a liquid-nitrogen subcooler, and a liquid pump L, which may be a rotary pump or a two-stage centrifugal `but is preferably a sim-ple plunger pump.

The cycle illustrated in Fig. 2 functions in the following manner. Air previously raised to a desired pressure, for example from 250 pounds to 1000 pounds 'per square inch gauge, and suitably puriiied, is introduced at I into coil HH of primary interchanger H, in which it is refrigerated by heat interchange with cold column products. A portion of the air stream is withdrawn at a medial point in the length of the coil .and passes .through vconduit F02 and a regulating and shut-olf valve i203 to an expansion engine I, by which its'pressure is reduced to .that carried 1in the high-pressure stage of the column, for example, 5 to .6 atmospheres absolute. The exhaust from the engine passes through conduit itil and a stop Vvalve m5 .to vthe lower -end vof column J.

The remainder of the air supply traverses the full length of coil Isl and vis likewise introduced 'into the 'lower end of the column through -conduits -lil and lili and expansion valve IDS. The air supplies passing through the expansion engine an-d the expansion valve may be Yproportioned-as desired, and it is possible, bysuitably controlling the initial air pressure, to dispense uwith the use ofthe expansion engine.

The crude oxygen separating in the highpressure-column stage .passes in the usual Inanner through conduit H39, expansion valve HU, and conduit H2 into the low-pressure-colurnn section, which is maintained-at a lower pressure, as for example from 2 to l0 lpounds per square inch gauge.

The `liquid nitrogen collecting in a pool H3 the upper 'end -of the vhigh-pressure section iflou/s through conduits H4 and HE, expansion valve H6, and conduit .i i? into the upper end of the low-pressure section. i

Low-pressure gaseous nitrogen separating in the upper section of the column ilows through conduit H and conduit i263 to `a jacket i2! surrounding the cold end of l'liquid pump L, from which it passes through conduit E22 to the shell v|23 of primary interchanger from which `it is ven-ted at |24.

Pure oxygen separating the low-pressure stage flows -through conduits 25, 126 and |28 to coil |29 of the primary interchanger, from which i-t is vented at i'. i

A desired portion or the lliquid, nitrogen collecting in pool H3 may be directed to the intake side of pump L vvthrough Valve i3! and conduit [32, the stream passing through a coil 1'33 in cooler K in which it is brought-to a desired lowered temperature in a manner described below. The subcoolevd liquid reaches the suction side Aof the pump through conduit l[Sli at `substantially the pressure maintained in the 'high-pressurecolumn section, is raised Iicy the pump to a desired higher pressure, and is discharged 'through ccnduitl 35 to afcoil lin the :primary i-interchanger. In this .coil the iliquid 1is vaporized'and brought substantially to atmospheric temperature Eby iheat liquid nitrogen stream Withdrawn from the pool ||3 is diverted from the conduit ||5 through a valve |54, and the diverted stream is conducted through a conduit |55, expansion valve |56 and conduit Id, to the shell |4| of the cooler K. After passing in heat exchange relation with the 'coil |33 the expanded diverted stream of liquid nitrogen flows from the shell |4| through a conduit |42 which connects with the conduit |22 to merge the diverted stream with the stream of gaseous product nitrogen on its Way to the main interchanger H.

The liquids separating in a gas-fractionating column are normally in equilibrium with the vapors above them. In pumping such liquids out of the column, fluid friction in the intake line 'and the weight of the intake valve of a reciprocating pump cause a drop in pressure between the point of supply and the barrel of the pump,

resulting in the evolution of vapor in reaching a new equilibrium at the lowered pressure. In the handling of these extremely volatile liquids, even a very slight drop in pressure causes sufiicient Vapor evolution to make it diiiicult or impossible to maintain the pump in smooth and continuous operation.

This difficulty is avoided in the operations herein described by cooling the liquid, in transit to the pumping step, to such degree that the 1inavoidable reduction in pressure is insufficient to bring the liquid to a new equilibrium at the reduced temperature. Assuming sulloient temperature reduction, this step is a positive cure for gas-locking and ensures continuous pump operation without short-stroking.

The extent to which the pumped liquid must be subcooled will, of course, depend on the pressure drop between the source of supply and the pump barrel in the particular installation, and

on the amount of heat conducted to the liquid u,

line through the insulation material surrounding it. Ordinarily a temperature reduction equivalent to that required to reduce the vapor pressure of the liquid by about one atmosphere will be suiiicient, but this iigure may be departed from rather widely with differences in actual pressure drop and in heat leakage into the intake line.

As nitrogen is the lowest-boiling component present in any appreciable quantity in air, and as no lower-boiling refrigerant is ordinarily available at the same absolute pressure, the cooling fluid must be expanded to a pressure below that under which the liquid passes to the pump.

The necessary pressure difference may be produced by applying artificially created pressure to the liquid flowing through the cooling coil, or by evacuating the chamber in which the coolant is ashed, but in the instant disclosure it is produced in the simplest possible manner by withdrawing the liquid to be pumped from the point in the fractionating cycle at which it already exists at a pressure materially above atmospheric, and by expanding the coolant to substantially atmospheric pressure, that is, to the lowest pressure at which it will ow at the required velocity through the interchangers in which its remaining refrigerative value is recovered. Y

The pressures existing in practice in the intermediate stage of the cycle of Fig. 1 or the highpressure stage of the cycle of Fig. 2 are ample to permit the cooling of even pure liquid nitrogen to a temperature of stability in the pump section by interchange with even what is normally the highest boiling product ofthe fractionation.

The method herein described is useful and valuable for the subcooling and pumping of all liqueed gases and gaseous mixtures rich in nitrogen and having vapor pressures approching 'those of nitrogen. Such gases, referred t0 Vin some of the claims at nitrogenous gases, include nitrogen in a state of more or less purity, atmospheric air, and oxygen-enriched air containing not less than about fty per cent nitrogen by weight.

I claim as my invention:

1. The method of transferring liquefied product of a fractionating operation, in which operation a mixture of compressed and cooled component gases having boiling points substantially below atmospheric temperature is expanded and the eliiuent of the expansion step subjected to the fractionating operation to produce a liqueiied higher boiling point fraction and a gaseous lower boiling point fraction at relatively low pressure, comprising forming as a step in the fractionating operation liquefied lower boiling point fraction at a pressure substantially equal to the vapor pressure cf the liqueed lower boiling point fraction, withdrawing a stream of the liqueed lower boiling point fraction from the fractionating operation, subcooling the stream of liquefied lower boiling point fraction by heat exchange with a relatively colder iiuid from the fractionating operation at a pressure lower than the pressure of the liquefied lower boiling point fraction, the last named pressure being such that the temperature of the liqueed lower boiling point fraction is reduced to a point below the boiling point of the liquefied lower boiling point fraction at the minimum momentary pressure reached in an ensuing pumping step, and pumping the subcooled liqueed lower boiling point fraction in liquid phase to a relatively high pressure.

2. The method of transferring liquefied product of a fractionating operation, in which operation a mixture of compressed and cooled component gases having boiling points substantially below atmospheric temperature is expanded and the eluent of the expansion step subjected to the fractionating operation to produce a liquefied higher boiling point fraction and a gaseous lower boiling point fraction at relatively low pressure, comprising forming as a step in the fractionating operation liqueed lower boiling point fraction at a pressure substantially equal to the vapor pressure of the liqueiied lower boiling point fraction, withdrawing a stream of the liqueed lower boiling point fraction from the fractionating operation, su'bcooling the stream of liquefied lower boiling point fraction by heat exchange with the gaseous lower boiling point fraction product from the fractionating operation to reduce the temperature of the liquefied lower boiling point fraction to a point below the boiling point of the liquefied lower boiling point fraction at the minimum momentary pressure reached in an ensuing pumping step, and pumping the subcooled liquefied lower boiling point fraction in liquid phase to a relatively high pressure.

3. The-method of transferring liquefied product .of a fractionating operation, in which operation a mixture of compressed and cooled component .gases hai/ing boiling points substantially below .atmospheric temperatur-.e is expanded and the effluent `of the expansion step subjected to the iracti'onating operation to produce a liquefied higher boiling point fraction and .a gaseous lower .boiling .point fraction lat v'relatively low pressure, comprising forming as :a step inthe -fractionati-ng :operation liquened .lower boiling point fraction :at Ia pressure substantially equal to the vapor ,pressure 4of the liquefied lower boiling point fraction, withdrawing a .stream of the liqueied .lower boiling point .fraction from the fractionat- Ving .oper-ation, .subcooling the stream of liquefied lower boiling point fraction by heat exchange with .expanded liqueed lower boiling point fraction from the fractionating yoperation to reduce the temperature of the liqueed lower boiling point fraction to a point below the boiling point of the liquefied lower boiling point fraction at the minimum :momentary pressure reached in an ensuing pumping step, ,and pumping the subcooled liqueiied lower boiling .point fraction in liquid 4phase .to .a relatively high pressure.

4. The method .of transferring liquened nitrogen .product .of a fractionating operation, in which .operation compressed and cooled air is ey.- panded and the .eliluent of the expansion stepl subjected to the iractionating operation to ,pro- -iduce liqueiied oxygen .higher boiling point frac tion and .the gaseous nitrogen lower boiling point fraction, comprising forming as a step in the fractionating `operation liquefied nitrogen lower boiling .point fraction at a pressure .substantially equal to the Vapor pressure of the liquefied nitrogen lower boiling point Sraction, withdrawing va ,stream of .the liqueed nitrogen lower boiling point fraction from the fractionating .operation, subcooling the stream of liquefied nitrogen lower 'boiling point fraction by heat exchange with a stream oi' Agaseous nitrogen .at a pressure lower than the pressure of the liquefied nitrogen lower boiling point fraction, the last-named pres .sure 'being such that the temperature of vthe liquefied nitrogen lower boiling point fraction is reduced to a point below the boiling point Aof the liqueed nitrogen lower boiling point fraction lat the minimumy moment-ary pressure reac'lied'in an ensuing pumping step, and pumping 'the subcooled liquefied nitrogen lower boiling point fraction in liquid phase to a relatively high pressure.

5. The method of `transferring liquefied nitrogen produ-ct or" a fractionating operation, in which operation compressed and cooled air is expanded andthe effluent of the expansion step subjected-to the vfractionating operation to produce liqueed oxygen higher boiling `point `fraction and gaseous nitrogen lower boiling point fraction, comprising lforming as a step Vin the racti'onating operation liquefied nitrogen lower boiling point fraction at a pressure .substantially equal to the vapor pres- 18 -sure Iof the liquefied .nitrogen lower boiling point fraction, withdrawing a stream of the liquefied nitrogen lower boiling :point vfraction from the -ffractionating operation, subcooling the withdrawn stream of liquefied nitrogen lower boiling point fraction by heat exchange with an exipanded stream 'of liqueed nitrogen lower boiling poi-nt `fraction .at a pressure lower than the pressure of the withdrawn stream of liquefied nitro- -gen .lower boiling point fraction, the last-named vpressure being such that the temperature of lthe `-liquefied nitrogen lower boiling :fraction .is rev'duced to a point below the boiling point of the vliqueiied ynitrogen lower boiling point fraction vat the mini-mum momentary pressure reached Jinan ensuing pumping step, and pumping the .sub- 'cooled liquefied nitrogen -lower boiling poi-nt fraction in liquid phase to a relatively high pressure.

@5. The method lof transferring liquefied nitrorgen product'oi a -ractionating oper-ation, in which operation compressed and 'cooled .air is expanded and the eiiluent of the expansion :step subjected to the fractionating 1operation Vto produce .liquelied-oxygen Ihigher `boiling point fraction and Egas- Aeous nitrogen lower boiling point fraction, comprising form-ing as astep lin :thefractionating operation liquefied nitrogen lower boiling point fraction at a pressure substantially equal to the vapor pressure Voi the liquefied nitrogen lower .boiling point fraction, withdrawing .a stream of the liquei'ed nitrogen lower boiling point Atraction from Ithe ractionating operation, .expanding .a stream of cold fluid from the fractionat-ingoperation to -a 'pressure lower than the pressure -oi lthe .lique- -ed nitrogen lower boiling point fraction, `subcooling the stream of liqueiied nitrogen lower boiling ,point fraetion by heat exchange with -the xpanded stream vof cold liquid from `the `fractionating opera-tion, the pressure oi the lexpanded stream being .such that the temperature -of Ythe liqueiied nitrogen lower boiling fraction is yreduced to a poi-nt below the boiling point of the liquefied nitrogen lower boiling point fraction at the minimum :momentary pressure reached in .an ensuing pumping step., .and pumping :the .subycooled 'liquefied nitrogen lower -boiling'pointffraction in liquid phase to a relatively -high pressure.

FRANK E. PAVLIS.

.References Cited in the .le of .this v.patent UNITEB STATES PATENTS Number Name Date 1,976,388 Eiche'lman Oct. '9, 1934 2,292,375 Hansen Aug. l'l, i942V 2,464,891 Rice Mar. 22, 1949 2,480,093 Anderson Aug. 23, 1949 2,480,094 Anderson Aug. 23, 1949 FOREGN PATENTS Number Country Date 495,795 VGermany Apr. 12, 1'9'30 500,544 Great Britain Feb. 7, T939 

